Dissolvable objects

ABSTRACT

A method of forming a dissolvable part of amorphous borate includes: preparing a mixture comprising one or more boron compounds and one or more alkali compounds, at least one of the one or more boron compounds and the one or more alkali compounds being hydrous; heating the mixture to a melting temperature for a predetermined time to melt the mixture and release water from the mixture to form an anhydrous boron compound that is moldable, wherein the amount of alkali compound being selected to achieve an alkali oxide content of between about 10 to 25%; with the anhydrous boron compound at a molding temperature, molding the anhydrous boron compound in a mold; and cooling the anhydrous boron compound to form a solid.

TECHNICAL FIELD

This relates to dissolvable objects made from anhydrous boron. Thedissolvable objects may be particularly useful for downholeapplications, such as selective fracturing tools, and may be used inother areas or industries where dissolvability is desired.

BACKGROUND

Boron is a known additive for glass, and can be used to adjust theproperties of a material. For example, boron can enhance the strengthand thermal properties of silicate glass when used as an additive, suchas in Pyrex™ glassware.

In other circumstances, boron can be used to make dissolvable glass,such as is described in U.S. Pat. No. 8,430,174 (Holderman et al.)entitled “Anhydrous boron-based timed delay plugs”, which describes theuse of dissolvable glass parts made from anhydrous boron to be used fordownhole applications in hydrocarbon-producing wells. In particular,dissolvable parts may be particularly useful in selective fracturingtools, which allow pressure to be selectively applied to a downholeformation in order to stimulate production. One type of selectivefracturing tool commonly used in the industry allows for multistagefracturing. These tools involve the use of different sizes of balls orparts that are pumped downhole to open a particular stage in thefracturing tool. Once the fracturing operation is complete, the ballsmust be removed, which is generally done by various methods such as bydrilling the balls out, backpressure suction, or backpressure releaseafter hold pressure is completed. The use of a dissolving ball wouldavoid the need to retrieve the balls and the associated costs anddifficulties.

SUMMARY

According to an aspect, there is provided a method of forming adissolvable part of amorphous borate, the method comprising the stepsof: preparing a mixture comprising one or more boron compounds and oneor more alkali compounds; heating the mixture to a melting temperaturefor a predetermined time to form an anhydrous boron compound that ismoldable, wherein the amount of alkali compound being selected toachieve an alkali oxide content of between about 10 to 25%; molding theanhydrous boron compound in a mold; an cooling the anhydrous boroncompound to form a solid.

According to another aspect, a majority of the alkali compound may be asodium compound.

According to another aspect, the anhydrous boron compound may be cooledto below a strain point of the solid at a minimum cooling rate that isinversely proportional to the square of one-half the thickness ordiameter of the solid being formed for a solid being cooled from morethan one side.

According to another aspect, the anhydrous boron compound may beinitially cooled by about 10° C. over at least a two hour period.

According to another aspect, the melting temperature may be about 400°C. or greater, and may be less than about 1000° C.

According to another aspect, the mold may be preheated to a temperaturethat is less than the melting temperature.

According to another aspect, the method may further comprise the step ofadding an additive to the mixture to adjust a dissolve rate of thesolid.

According to an aspect, the mixture may comprise one or more additiveshaving at least one of corrosion resistant and antibacterial properties.

According to another aspect, the mixture may comprise equal measures byweight of boric acid and disodium octaborate tetrahydrate.

According to another aspect, the mixture may comprise one or morecompounds selected from a group consisting of: hydrated alkalineborates, hydrated nonalkaline borates, refined borate, mineral borate,sodium borate, sodium metaborate, disodium octaborate tetrahydrate,borax, boric acid, copper borate, lithium borate, potassium borate,silver borate, zinc borate, boron halide, colemanite, kernite,probertite, tincal, and ulexite.

According to another aspect, the method may further comprise the step ofselecting the amount of the one or more hydrous compounds in the mixtureto adjust a dissolve rate of the solid.

According to another aspect, the mixture may be heated to a temperatureof between about 400° C. and about 1050° C., or to a temperature ofbetween about 800° C. and about 900° C.

According to another aspect, the mixture may be cooled to below theglass transition temperature over a period of between 1 and 4 hours.

According to another aspect, the part is a sphere with a diameter ofabout 3 inches or less.

According to another aspect, the method may further comprise the step ofadjusting the dissolvability by adding an additive selected from a groupI, group II, and group III metal on the periodic table.

According to another aspect, the method may further comprise the step ofadjusting the dissolvability by adding an additive selected from a groupconsisting of copper, manganese, molybdenum, zinc, calcium, and silver.

According to an aspect, there is provided a dissolvable borate partprepared by a process comprising the steps of: creating a mixture of oneor more borates and one or more alkalis; melting the mixture by heatingto a temperature of between about 700° C. and about 1000° C. to form amoldable fluid, the moldable fluid comprising an alkali oxide content ofbetween about 10 mol % to about 25 mol % and a boric oxide content ofbetween about 75 mol % and about 90 mol %; molding the moldable fluid ina mold; cooling the moldable fluid to form a solid, the anhydrous boroncompound being cooled to below a strain point of the solid over a periodof at least 1-4 hours ; and removing the cooled solid from the mold.

According to another aspect, the mixture is melted at a melt temperatureof between about 800° C. and about 900° C.

According to another aspect, the melted mixture may be poured into amold that has a mold temperature that is less than the melt temperature.

According to another aspect, the moldable fluid may comprise an additiveselected from a group consisting of copper, zinc, and silver.

According to another aspect, the dissolvable borate part may furthercomprise an additive selected from a group I, group II, and group IIImetal on the periodic table to the mixture.

According to another aspect, the dissolvable borate part may furthercomprise an additive selected from a group consisting of copper,manganese, molybdenum, zinc, calcium, and silver.

According to another aspect, the anhydrous boron compound may comprisebetween about 12-20 mol % Na₂O and between about 80-88 mol % B₂O₃, orabout 18 mol % Na₂O and about 82 mol % B₂O₃.

According to another aspect, the part may be a sphere with a diameterequal to or less than about 3 inches and is cooled to the ambienttemperature over a period of about 1-3 hours.

According to an aspect, there is provided a dissolvable borate part,comprising an anhydrous borate compound that is molded to form an objecthaving a Young's Modulus of at least 30 GPa, the anhydrous boratecompound comprising between about 75 and 90% mol of B₂O₃; and betweenabout 10 and 25% mol of one or more alkali oxide.

According to another aspect, the dissolvable borate part of claim maycomprise about 82 mol % of B₂O₃ and about 18 mol % of the one or morealkali, and the object has a Young's Modulus of at least 40 GPa.

According to another aspect, a majority of the one or more alkali maycomprise Na₂O.

According to another aspect, the anhydrous borate compound may besubstantially free of silicate.

According to an aspect, there is provided a method of forming adissolvable part of amorphous borate, the method comprising the stepsof: preparing a mixture comprising one or more boron compounds and oneor more alkali compounds, at least one of the one or more boroncompounds and the one or more alkali compounds being hydrous; heatingthe mixture to a melting temperature for a predetermined time to form amoldable anhydrous boron compound; molding the anhydrous boron compoundin a mold; and cooling the anhydrous boron compound to form a solid.

According to another aspect, the amount of alkali compound may be suchthat the alkali oxide is between 10-25%.

According to another aspect, the anhydrous boron compound may comprisebetween about 12-20 mol % Na₂O and between about 80-88 mol % B₂O₃, orabout 18 mol % Na₂O and about 82 mol % B₂O₃.

According to another aspect, the mixture may be cooled to below a strainpoint of the solid over a period of at least 1-4 hours.

According to an aspect, there is provided a method of forming adissolvable part of amorphous borate, the method comprising the steps ofproviding a boron mixture comprising one or more hydrated boroncompounds, melting the boron mixture by heating the boron mixture to atarget temperature, applying heat to release water from the boronmixture to form a moldable anhydrous boron compound, molding themoldable anhydrous boron compound in a mold, the mold being maintainedat a mold temperature that is above an ambient temperature, and coolingthe moldable anhydrous boron compound to form a solid by maintaining themold at the mold temperature for at least 1 minute and then cooling themoldable anhydrous boron compound to the ambient temperature over aperiod of at least 4 hours.

According to another aspect, the method may further comprise the step ofadding an additive to the mixture to adjust a dissolve rate of thesolid.

According to another aspect, the mixture may comprise one or moreadditives having at least one of corrosion resistant and antibacterialproperties.

According to another aspect, boron mixture may comprise equal measuresof boric acid and disodium octaborate tetrahydrate.

According to another aspect the boron mixture may comprise one or morecompounds selected from a group consisting of: hydrated alkalineborates, hydrated nonalkaline borates, refined borates, mineral borates,sodium borate, sodium metaborate, disodium octaborate tetrahydrate,borax, boric acid, copper borate, lithium borate, potassium borate,silver borate, zinc borate, boron halide, colemanite, kernite,probertite, tincal, and ulexite.

According to another aspect, the method may further comprise the step ofselecting the amount of the hydrated boron compounds in the boronmixture to adjust a dissolve rate of the solid.

According to another aspect, the mixture may be heated to a temperatureof between about 340° F. and about 1900° F.

According to another aspect, the mixture may be heated to a temperatureof between about 1500° F. and about 1700° F.

According to another aspect, the mixture may be poured into a moldhaving a temperature of between about 300° F. and about 1200° F.

According to another aspect, the mixture may be poured into a moldhaving a temperature of between about 600° F. and about 900° F.

According to another aspect, the mold may be maintained between 600° F.and 900° F. for at least 3 minutes prior to cooling the part.

According to another aspect, the boron mixture may be cooled to form thesolid over a period of between 8-10 hours.

According to another aspect, the mold may be maintained at the moldtemperature for between 1 and 15 minutes.

According to another aspect, the part may be a sphere with a diameter ofabout or less than 2 inches.

According to another aspect, the part may be cooled over a period of atleast 22 hours and the part is a sphere with a diameter of about orgreater than 4 inches.

According to an aspect, there is provided a dissolvable borate partprepared by a process comprising the steps of creating a mixture of oneor more borates, melting the mixture by heating to a temperature ofbetween about 340° F. and about 1900° F. to form a moldable fluid,molding the moldable fluid in a mold, the mold being maintained at amold temperature that is above an ambient temperature, cooling themoldable anhydrous boron compound to form a solid by maintaining themold at the mold temperature for at least 1 minute, removing thesolidified mixture from the mold, and cooling the mold to the ambienttemperature over a period of at least 4 hours.

According to another aspect, the mixture may be heated to a temperatureof between about 1500° F. and about 1700° F.

According to another aspect, wherein the melted mixture may be pouredinto a mold that has a mold temperature of between 600° F. and 900° F.

According to another aspect, the mold may be maintained at the moldtemperature for about 1-15 minutes after receiving the melted mixtureand prior to cooling the heated mixture.

According to another aspect, the heated mixture may be cooled over aperiod of 8-10 hours after being removed from the mold.

According to another aspect, creating a mixture may further compriseadding a corrosion inhibitor selected from a group consisting of copper,zinc and silver.

According to another aspect, creating a mixture may further compriseadding an additive selected from a group I, group II, and group IIImetal on the periodic table.

According to another aspect, creating a mixture may further compriseadding an additive selected from a group consisting of copper, zinc, andsilver.

According to another aspect, creating a mixture may further compriseadding an antibacterial agent selected from a group consisting ofsilver, copper, zinc and silver.

According to another aspect, creating a mixture may further comprise thestep of adjusting the dissolvability by adding an additive selected froma group I, group II, and group III metal on the periodic table.

According to another aspect, creating a mixture may further comprise thestep of adjusting the dissolvability by adding an additive selected froma group consisting of copper, manganese, molybdenum, zinc, calcium, andsilver.

According to another aspect, the mixture may comprise 25-75% by weightof hydrated boric acid and between 25-75% by weight of disodiumoctaborate tetrahydrate.

According to another aspect, the mold may be maintained at the moldtemperature for between 1 and 15 minutes.

According to another aspect, the part may be a sphere with a diameter ofabout or less than 2 inches and is cooled to the ambient temperatureover a period of about 8 to 10 hours.

According to another aspect, the part may be cooled over a period of atleast 22 hours and the part is a sphere with a diameter of about orgreater than 4 inches.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the followingdescription in which reference is made to the appended drawings, thedrawings are for the purpose of illustration only and are not intendedto be in any way limiting, wherein:

FIG. 1 is a schematic view of a crucible filled with a borate powdermixture.

FIG. 2 is a schematic view of a crucible with a melted borate mixturebeing poured into a mold.

FIG. 3 is a graph comparing density and percentage of sodium oxide.

FIG. 4 is a graph comparing different relative amounts of sodium oxideand boric oxide.

FIG. 5 is a graph of the isothermal viscosities of alkali borate melts.

FIG. 6 is a graph showing an example of a cooling schedule.

DETAILED DESCRIPTION

There will now be described a preferred method and composition forforming objects made from a dissolvable material, namely, a properlyprepared anhydrous boron glass. It will be understood from thedescription below that, while all permutations and combinations offeatures are not described, the features described herein may becombined in any logical and predictable manner by a person of ordinaryskill.

The method to form the dissolvable parts begins by assembling anappropriate mixture of components 12. Referring to FIG. 1, thesecomponents 12 will generally be in powder form as this allows thecomponents to be easily measured and mixed together, although thecomponents may be provided in other convenient states.

In one example, the components of the mixture includes refined ormineral sources of boron. This may include hydrated alkaline andnonalkaline, mineral or refined borates, such as sodium borate, disodiumoctaborate tetrahydrate (DOT), sodium metaborate, borax, boric acid,metallic borate compounds such as copper, lithium, potassium, silver,and zinc, and other boron halide materials. Mineral borates may also beused, such as colemanite, kernite, probertite, tincal, ulexite, Neobor™(available from Borax), etc. In addition, multiple sources of boron maybe mixed together in equal or disproportionate amounts.

The mixture preferably has at least one hydrous compound, such that themixture is a hydrous mixture when melted. It has been found that thisassists in forming a moldable mixture once heated, and in some cases,the hydrous form of the compounds are less expensive and easier toprocure. As will be apparent from the discussion below, other mixturesthat are anhydrous and that allow the desired end composition to beachieved may also be used.

Some examples of formulations for making the dissolvable materialinclude: two-part mixtures made up of boric acid and DOT with about25-50 wt % and 50-75 wt % of each component being present in themixture; 95 wt % boric acid and 5 wt % DOT; 2.9 wt % copper hydroxide,7.1 wt % boric acid, and 90 wt % DOT; and 10 wt % copper hydroxide, 10wt % boric acid, and 80 wt % DOT. In another example, one or moresources of boron are mixed with one or more sources of alkali metal,such as Na, K, Li, or Ca, preferably one of which is in a hydrous form.For example, DoT is a hydrous form of Na, although Na may also be addedin other forms, such as NaOH, or a mixture of hydrous and non-hydrousforms. In addition, boron may be added in hydrous or non-hydrous forms,or combinations thereof. Those of ordinary skill will be able to chooseappropriate amounts of each component required to achieve the desiredend composition, described below.

In one example, the final composition, once heated, will have a formulaof between about 10-25 mol % Na₂O and between about 75-90 mol % B₂O₃. Ina preferred embodiment, there may be about 18 mol % Na₂O and 82 mol %B₂O₃. It will be understood that there may be some inherent variation inthe actual chemical formula due to imperfect mixing or distribution, orinexact variations. For example, in terms of boric oxide the finalcompound may have small amounts of both hydrous and anhydrous states ofboric oxide. In addition, there may be trace amounts of impurities suchas fluoride, sulphate and iron. The amount of variation will depend onthe source, and the tolerances for a particular application. Similarconsiderations may also apply to other components, including alkalis.

As discussed, a portion of Na may be replaced with other alkalis toachieve a desired set of physical characteristics, and other additivesmay also be present to introduce other elements that may perform otherfunctions. It will be apparent that different additives will affect theproperties in different ways, and care must be taken to ensure theproperties are suitable for the intended use. As can be seen, thedissolvable material is preferably free of silicates, which is the basematerial used in forming most common types of glass.

Pure B₂O₃ has a T_(g) (glass transition temperature) of about 260° C.due to the layered structure composed of strong BO₃ triangles, whichform a layer type of structure similar to that in mica. Bonding betweenthe layers is very weak, allowing a measurable viscosity at lowtemperature and a high thermal expansion. Adding Na₂O provides an extraoxygen which is used by a B to become 4 fold coordinated, with the extraoxygen providing a bridge between 2 BO₃ layers. This reduces mobility ofthe structure and causes a decrease in thermal expansion and an increasein T_(g.). For example, one example that contains 12 mol % Na₂O has aT_(g) of 380° C. and a linear expansion of 160° C. Young's Modulus ofthe glass will increase with the initial additions of alkali as expecteddue to the increase in strong bonds between layers. Since the layers areincreasingly bridged, one would expect the strength to increase as well,and in fact, the “theoretical” strength, which is determined primarilyby Young's Modulus and Fracture Surface Energy, does increase. However,there are also other important attributes to consider as well.

The most logical way to increase modulus is by an increase in Nacontent. However, a mixed alkali composition could also be used, such asby adding some additional Li, or K, or Ca. It was found that adding CaOto the mixture in the amounts of 12 mol % CaO, 12 mol % Na₂O to 76 mol %B₂O₃ did not give favorable results, due to the high melting point ofthe composition. Adding Na₂O alone still increases the Modulus but witha much smaller effect on viscosity (T_(g)) and melting point. This isshown in FIG. 3.

The melting point of pure B₂O₃ is seen to be about 600° C., while adding12 mol % melts near 800° C., and the proposed 18-Na₂O melts at 825° C.Commercial glasses are typically melted at a temperature where theviscosity is about 100 Poise. The viscosity of the 12 and 18 mol % Na₂Omelts are relatively low even at 800° C., and fairly rapid fining (i.e.bubble removal) can be expected. The 12 mol % glass has a viscosity of50 poise and the 18 mol % glass has a viscosity of a little less than100 poise. At 900° C., the 18-Na₂O glass has a viscosity of about 20Poise, much lower than generally used for glass melting. Thus, bothglasses can likely be melted at about 850° C. (1550° F.), or just abovethe liquidous phase, with rapid homogenization. It has been found thatsubstantially all water is removed after 1 hour at this temperature andthat the glasses formed are bubble free. Any bubbles that may appear inthe poured samples after these periods are likely due to those whichform at the glass/air/crucible interfaces and which flow out with theglass as it is poured.

It should be noted that when using stainless steel crucibles forchemical reactions and oxidation, temperatures above about 850° C.should be avoided. As the viscosity data indicate that the highertemperatures are not needed, it is possible to use stainless steelcrucibles if care is taken to avoid exceeding the maximum temperature,at which point bubble formation may occur due to corrosion.

If a lower viscosity is desired, it may be possible to add a smallamount of Li₂O in place of some of the Na₂O (see FIG. 4).

Once the mixture is formed, it is then melted. Referring to FIG. 2, thisis preferably done by placing the mixture in a heated crucible 14 orother vessel suitable for melting at high temperatures. Mixture 12 ismelted to form a boron mixture in a liquid state and is heated untilmixture 12 is in an appropriate state to be molded. While the mixture isinitially hydrous, sufficient heat is applied over a sufficient periodof time to remove the water in the mixture such that the mixture isanhydrous once it is molded in mold 16. Preferably, the melting timewill be less than about 3 or 4 hours, and preferably 1 or 2 hours, inorder to release any water in the mixture. The actual temperature andlength of time may vary depending on the size of crucible 14 andcomposition of the mixture, and can be determined by routine testing. Itshould be noted that, if the mixture is heated too long, the compositionmay begin to degrade by releasing boron, leaving an excess of oxygen.

In one example, adequate results were achieved by mixing equal portionsof boric acid and DOT and heating the mixture to about 800° C. (1500°F.). Generally speaking, the mixture may be heated to between 400° C.and 1050° C. (750° F. and 1900° F.) and more preferably between 800° C.and 900° C. (1500° F. and 1700° F.). Once heated properly to form ananhydrous, moldable mixture, mixture 12 can then be placed in a mold 16,such as by pouring as shown. The details of filling molds with mixture12 as well as the types of molds that may be used, are known in the artand will not be described in more detail here. It will be understoodthat FIG. 2 depicts a generic example of a molding process.

If the mixture is not heated properly, the resulting glass may beweakened and may affect the consistency of the dissolvingcharacteristics of the part. It is believed that the weakness may resultfrom water being present in the mixture, and that the effect can bereduced by properly heating the mixture such that the mixture is moldedin an anhydrous state. Care must also be taken not to heat mixture toolong, as boron may be released from the mixture and release oxygen intothe mixture, which may weaken the glass once cooled, or the mixture mayset such that it cannot easily be poured or molded. It has been foundthat a solid anhydrous compound cannot be easily melted and re-moldedwithout a significant increase in the expenditure of energy required.

Prior to receiving mixture 12, mold 16 is preferably heated to atemperature around the annealing point of the resulting glass, which isessentially the glass transition point of the glass being formed. Thisis useful because stresses cannot be developed in the glass above thistemperature. The heat also liberates any water that may be adsorbed tothe surface of the mold prior to adding the mixture. In addition toheating the mold, a mold release agent is also preferably applied to themold surface to prevent sticking as is commonly done in industry. Theagent is typically a boron nitride material and is widely available.

In one example, adequate results were achieved by pouring mixture 12 ata minimum temperature of 800° C. (1500° F.) into a mold that has beenheated to about 350° C. (700° F.). The mold may be at a temperature ofbetween 200° C. (400° F.) and 800° C. (1500° F.). Preferably, the moldtemperature is between 300° C.-500° C. (600° F.-900° F.). The actualtemperature will depend on the preferences of the user and theproperties of the particular mixture 12 being used. However, it has beenfound that if the mold temperature is below the minimum, the structuralproperties of the resulting are weakened, and may not be sufficientlysound, depending on the intended purpose. On the other hand, if the moldtemperature is too hot, it has been found that the mixture adheres tothe metal of the mold.

Once mold 16 is filled with mixture 12, mold 16 and mixture 12 arecooled until mixture 12 is a solid and can be handled. The time afterwhich the mixture can be removed from the mold will vary by the size andmass of object being molded. Care must be taken during this step as afailure to cool the mixture properly may result in cracking, splitting,deformation or premature failure of the object. Once sufficientlycooled, the now solidified part may then be removed from mold 16 tocontinue conditioning to form a stable solid. In one example, whenforming spheres that were about 2 inches in diameter, a suitable coolingprocess involved a cooling rate of about 0.1° C./minute for the first 2hours to reduce the risk of permanent residual stresses. It will beunderstood that some residual stress may be allowed and possiblydesired, since the surface would be under a slight compression, whichmay increase its strength. Once the initial cooling step has occurred,mixture 16 may be cooled more quickly while still taking care to coolthe glass at a safe rate, as the resulting glass is still subject tothermal expansion.

During the cooling phase, the glass may freeze into different structureswith different densities and other properties, depending on how fast itis cooled. For example, the density will be lower if the glass israpidly cooled, and higher if it is cooled more slowly. This should betaken into account for thicker materials, which may have a variabledensity from the surface to the center.

A variable density may be beneficial in some circumstances, and mayincrease the strength of the material. Considering a sphere, if theouter surface is cooled more rapidly than the inside, the outer surfacewill have a lower density than the inside, which will cause tension asthe inner area tries to shrink, placing the outer surface undercompression. However, if the resulting solid is to be cut or sectioned,the internal stresses could cause weaknesses or failure. For example, ifa sphere is formed and has a “tail” (e.g. from the rod formed during themolding process) that must be removed, this will be a source of weaknessin the structure.

When cooling mixture 12, it may shrink. This may cause problems in someapplications, such as when manufacturing plugs for a selectivefracturing operation, which rely on strict tolerances. Shrinkage may beaddressed by changing the mold design, or by using a glass compositionthat does not shrink as much. For example, a suitable amount of anadditive such as Sodium oxide (Na₂O) may reduce the amount of shrinkagethat occurs. While the glass will still be subject to thermal expansion,it has been found that the coefficient may be reduced by about 30 mol %by increasing the amount of Na₂O in the mixture from 12 mol % to 18 mol%.

Referring to FIG. 6, a typical cooling schedule for a glass is shown,where d is the thickness of the glass, or half thickness if the glass iscooled from both sides, and α is the linear thermal expansioncoefficient (e.g. 10×10⁻⁷/C). As will be understood, α will depend onthe composition of the glass, and will be impacted by the components,and in particular, the amount of alkali. Region A is the “pouring”region, where the temperature may go up or down, depending on therelative temperatures of the mixture and the mold. The heating rate ofregion A may be described by the expression: 839/[αd²]° C./min.

Region B is a hold time at the temperature that is slightly more, suchas about 5° C. higher, than the annealing point, which is approximatelyequivalent to the T_(g) for the glass. The time in region B may bedescribed by the expression 97d min.

Region C is the “slow cool” stage and is the critical region forrelieving stresses. The rate in region C may be described by theexpression: 42.6/[αd²]° C./min. In the general formula, the coefficient(i.e. 42.6 in this case) relates to the Arrhenius equation, and willdepend on the specific temperatures involved. Measurement of the StrainPoint and Annealing Point may be difficult but can be estimated from theT_(g) data obtained using Differential Scanning calorimetry (DSC). Themeasured T_(g) for an 11 mol % composition was about 380° C., and theactivation energy was about 575 kJ/mol. For an 18 mol % composition, theT_(g) was about 450° C. and the activation energy was about 780 kJ/mol.Using these numbers, and assuming an Arrhenius behaviour for viscosityin the temperature range of interest, e.g. η=η_(o) exp(−ΔE_(act)/RT), itis possible to extrapolate from th T_(g) where viscosity is 10¹² poiseto the annealing point where viscosity is 10¹³ poise and to the strainpoint where viscosity is 10^(14.5) poise. The temperature of the strainpoint is calculated to be 360° C. Based on a 380° C. initialtemperature, a 360° C. final temperature, and an expansion coefficientof 104×10⁻⁷, it is calculated that the cooling rate between these twotemperatures for minimum stress generation should be =42.6/(104)(halfthickness). For a 3 inch (7 cm) diameter ball, the rate would be about0.03° C./min or about 2° C./hour. While this is a minimum rate, theactual rate may be faster, such as at least 0.1° C./min, while stillproducing a sufficiently stable structure.

Region D is a “fast cool” stage, which is about twice as fast as theslow cool stage, and region E is the “final cool” stage, which may be upto 10 times the speed of the slow cool stage.

The above considerations are relative to the preferred embodiment with18 mol mol % Na₂O. If a higher alkali content is used, or a differentalkali or alkali mixture is used, the thermal expansion may decrease orincrease, and allow it to be cooled more rapidly, or require a slowercooling process than what is described above.

Another alternative to the cooling schedule described above is to coolthe glass rapidly from the pour temperature and then reheat the samplesfor a “proper” anneal following the Corning formula

Applications

When used in a selective fracturing tool, the parts are preferablyspherical in shape and may be molded with different diameters.Historically, different shapes have been proposed for use in selectivefracturing tools. However, it has been determined that spheres work bestin the present sleeve activation. Other shapes may also be used ifdesired by the user, or if the resulting parts are intended to be usedfor a different purpose. For example, rather than being used as a plug,the seal seats or a component within the seat may be made from thedissolvable material, such that the seat dissolves, or is released forretrieval after a desired period of time. Alternatively, the parts maybe used on other pats that are inserted downhole, but that only have atemporary use, or are difficult to retrieve. By using dissolvable parts,the tools may entirely dissolve, or may be released, making them easierto retrieve at a desired time.

In order to be used in a downhole environment, the parts must besufficiently strong to withstand the typical pressures that will beencountered downhole. If the parts are too brittle, the parts mayshatter before they are able to open the stage or maintain the sealuntil the part dissolves. The specific requirements are known in the artand will not be discussed further.

As stated previously, parts are designed to be dissolvable, although therate at which the parts dissolve will depend on the temperature,pressure, and fluids encountered downhole. It is possible to adjust thedissolve rate of the part to be either longer or shorter, depending onthe preferences of the user and the intended use. In the case ofdownhole operations, the rate of dissolution must be sufficiently longto allow the fracturing operation to be completed, but not so long thatfurther operations are delayed while the operator waits for the parts todissolve. The dissolve rate may be modified based on the selection ofthe original boron compounds used to create the mixture and the relativeproportions of the components in the mixture. Generally, for the mixturedescribed herein, there is about a ⅛″ loss in 4 hours. The dissolve ratemay also be adjusted through the use of additives, such as the group I,group II, or group III metals on the periodic table. For example, theadditive may be selected from copper, manganese, molybdenum, zinc,calcium, and silver, each of which may be introduced as a compound. Inaddition, additives may also be included to deliver compounds downholeas the parts dissolve. For example, the spheres may be used as adelivery method for compounds that act as corrosion inhibitors, such as,for example, copper, zinc, and silver. As will be understood by oneskilled in the art, other elements known to exhibit this function mayalso be selected as a corrosion inhibitor. In addition, the additivesmay deliver antibacterial compounds such as silver, copper, and zinccompounds or any metalloid that exhibits antibacterial properties.Additives with other properties may also be included in the parts. Inaddition to the use of additives, the parts may have one or morecoatings that delay the dissolution of the part, or deliver desiredcompounds downhole. For example, the coating may be designed to react toa particular event or chemical, such that the parts may be stable untila condition is selectively altered, which triggers its dissolution.

In addition to the selective fracturing tools, the anhydrous boron glassdescribed herein may be used in other circumstances. For example, theglass may be used to make tools or equipment that are either difficultto retrieve or may be lost during use. In these circumstances, the partsmay be made from a glass described herein, and left to dissolve withouthaving to be retrieved.

In this patent document, the word “comprising” is used in itsnon-limiting sense to mean that items following the word are included,but items not specifically mentioned are not excluded. A reference to anelement by the indefinite article “a” does not exclude the possibilitythat more than one of the element is present, unless the context clearlyrequires that there be one and only one of the elements.

The scope of the following claims should not be limited by the preferredembodiments set forth in the examples above and in the drawings, butshould be given the broadest interpretation consistent with thedescription as a whole.

1. A method of forming a dissolvable part of amorphous borate, themethod comprising the steps of: preparing a mixture comprising one ormore boron compounds and one or more alkali compounds, wherein at leastone of the compounds is a hydrous compound; heating the mixture to amelting temperature of between about 700° C. and about 1000° C. anddwelling at the melting temperature for a sufficient time to release thewater in the mixture and form an anhydrous boron compound that is amoldable fluid, wherein the amount of alkali compound selected achievesan alkali oxide content in the moldable fluid of between about 10 mol %to 25 mol %; molding the anhydrous boron compound in a mold; and coolingthe anhydrous boron compound to form a solid to below a strain point ofthe solid over a period of at least 1-4 hours.
 2. The method of claim 1,wherein a majority of the alkali compound is a sodium compound.
 3. Themethod of claim 1, wherein the anhydrous boron compound is cooled tobelow a strain point of the solid at a minimum cooling rate that isinversely proportional to the square of one-half the thickness ordiameter of the solid being formed for a solid being cooled from morethan one side.
 4. The method of claim 1, wherein the anhydrous boroncompound is cooled from the melting temperature by about 10° C. over atleast a two hour period.
 5. The method of claim 1, wherein the meltingtemperature is about 400° C. or greater.
 6. The method of claim 6,wherein the melting temperature is less than about 1000° C.
 7. Themethod of claim 1, wherein the mold is preheated to a temperature thatis less than the melting temperature.
 8. The method of claim 1,comprising the step of adding an additive to the mixture to adjust adissolve rate of the solid.
 9. The method of claim 1, wherein themixture comprises one or more additives having at least one of corrosionresistant and antibacterial properties.
 10. The method of claim 1,wherein the mixture comprises equal measures by weight of boric acid anddisodium octaborate tetrahydrate.
 11. The method of claim 1, wherein themixture comprises one or more compounds selected from a group consistingof: hydrated alkaline borates, hydrated nonalkaline borates, refinedborate, mineral borate, sodium borate, sodium metaborate, disodiumoctaborate tetrahydrate, borax, boric acid, copper borate, lithiumborate, potassium borate, silver borate, zinc borate, boron halide,colemanite, kernite, probertite, tincal, and ulexite.
 12. The method ofclaim 1, further comprising the step of selecting the amount of the oneor more hydrous compounds in the mixture to adjust a dissolve rate ofthe solid.
 13. The method of claim 1, wherein the mixture is heated to atemperature of between about 800° C. and about 900° C.
 14. The method ofclaim 1, wherein the molded anhydrous boron compound is cooled to belowa glass transition temperature of the solid over a period of between 1and 4 hours.
 15. The method of claim 1, wherein the molded anhydrousboron compound is cooled at a rate of less than 0.1° C./minute.
 16. Themethod of claim 1, wherein the part is a sphere with a diameter of about2 inches or more.
 17. The method of claim 1, further comprising the stepof adjusting the dissolvability by adding an additive selected from agroup I, group II, and group III metal on the periodic table.
 18. Themethod of claim 1, further comprising the step of adjusting thedissolvability by adding an additive selected from a group consisting ofcopper, manganese, molybdenum, zinc, calcium, and silver. 19-36.(canceled)